Method and apparatus for interleaving sequence elements of an ofdma synchronization channel

ABSTRACT

A method and apparatus is provided for transmitting an orthogonal frequency domain multiple access (OFDMA) signal including a synchronization channel signal transmitted including a plurality of sequence elements interleaved in time and frequency. The synchronization channel signal sequence elements enable an initial acquisition and cell search method with low computational load by providing predetermined time domain symmetry for common sequence elements in OFDMA symbol periods for OFDMA symbol timing detection and frequency error detection in an OFDMA system supporting multiple system bandwidths, both synchronized and un-synchronized systems, a large cell index and an OFDMA symbol structure with both short and long cyclic prefix length.

RELATED APPLICATION

This application is a divisional application of co-pending U.S. patentapplication Ser. No. 13/052,777, filed on Mar. 21, 2011, which is acontinuation of U.S. patent application Ser. No. 11/351,275 (now U.S.Pat. No. 7,911,935), filed on Feb. 8, 2006, all of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to wireless communicationsystems, and more particularly relates to a method and apparatus in anorthogonal frequency division multiple access (OFDMA) system forinterleaving sequence elements of an OFDMA synchronization channel.

BACKGROUND OF THE INVENTION

In a wireless communication system which includes a number of basestations or cells, an initial task for a wireless communication deviceis to recognize and acquire the signals transmitted from the cells.Another primary task is to search the cells to determine which cell isthe best for establishing communication with. As more and more complexsignaling systems are developed, these important tasks become moredifficult and more time-consuming. Recently, orthogonal frequencydivision multiple access (OFDMA) signaling systems have been proposed.The OFDMA systems are scalable bandwidth systems designed to work indifferent bandwidths. In addition, the OFDMA systems utilize amulti-carrier modulation approach having, perhaps, hundreds ofsubcarriers within a narrow (e.g., 5 MHz) frequency range. While thescalability of OFDMA systems facilitates the introduction and expansionof such systems, the complexity of OFDMA systems must nevertheless allowfor signal acquisition by OFDMA wireless communication devices in atimely manner for quick activation and seamless transition from cell tocell. To enable initial acquisition and cell search, a synchronizationchannel is provided. However, the length and complexity of thesynchronization channel signal increases with the number of cells andthe complexity of system design and signal propagation solutions.

Thus, what is needed is a method and apparatus for interleaving sequenceelements of a synchronization channel signal for improved initialacquisition and cell search. Furthermore, other desirable features andcharacteristics of the present invention will become apparent from thesubsequent detailed description of the invention and the appendedclaims, taken in conjunction with the accompanying drawings and thisbackground of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 is a diagram of a wireless communication system in accordancewith an embodiment of the present invention;

FIG. 2 is a diagram of a frame structure of an orthogonal frequencydomain multiple access (OFDMA) signal in accordance with an embodimentof the present invention;

FIG. 3 is a diagram of a frame structure of an OFDMA signal inaccordance with an alternate embodiment of the present invention;

FIG. 4 is a diagram of the signal channel bandwidth occupation inaccordance with an embodiment of the present invention;

FIG. 5 is a diagram of the resource block mapping of the synchronizationchannel in accordance with an embodiment of the present invention;

FIG. 6A is a diagram of the synchronization channel sequence assignmentin accordance with an embodiment of the present invention;

FIG. 6B is a diagram of the synchronization channel sequence assignmentin accordance with an alternate embodiment of the present invention;

FIG. 6C is a diagram of the synchronization channel sequence assignmentin accordance with yet another alternate embodiment of the presentinvention;

FIG. 7 is a diagram of the sub-carrier mapping of the synchronizationchannel signal in accordance with the embodiment of the presentinvention;

FIG. 8 is a block diagram of a base station of the communication systemof FIG. 1 in accordance with the embodiment of the present invention;

FIG. 9 is a flowchart of the base station synchronization channelsignaling of the base station of FIG. 8 in accordance with theembodiment of the present invention;

FIG. 10 is a block diagram of a wireless communication device of thecommunication system of FIG. 1 in accordance with the embodiment of thepresent invention; and

FIG. 11 is a flowchart of the initial activation and cell search of thewireless communication device of FIG. 10 in accordance with theembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with an embodiment of the present invention, a method in awireless communication system includes the step of transmitting anorthogonal frequency domain multiple access (OFDMA) signal including asynchronization channel signal transmitted within a localized portion ofa bandwidth of the OFDMA signal, the synchronization channel signalhaving predetermined time domain symmetry within the localized portionof bandwidth and including information for providing at least partialcell identification information. In addition, a method in a wirelesscommunication system in accordance with an embodiment of the presentinvention includes the step of transmitting an OFDMA signal including asynchronization channel signal, the synchronization channel signalincluding a plurality of synchronization channel signal sequenceelements and the OFDMA signal including a plurality of subcarriers and aplurality of OFDMA symbol periods, wherein the plurality ofsynchronization channel signal sequence elements are distributed amongeither or both of the plurality of subcarriers and a plurality of timeintervals such as the plurality of OFDMA symbols periods.

Also, in accordance with an embodiment of the present invention, amethod for receiving OFDMA signals includes the steps of isolating aportion of a bandwidth of the OFDMA signals which includes asynchronization channel signal, detecting a position of thesynchronization channel within the portion of the bandwidth of the OFDMAsignals, and decoding the synchronization channel signal to derive atleast partial cell identification information therefrom.

Referring to FIG. 1, an orthogonal frequency division multiple access(OFDMA) wireless communication system 100 in accordance with anembodiment of the present invention includes a plurality of basestations 110 and a wireless communication device 120. The plurality ofbase stations 110 communicate with the wireless communication device 120via OFDMA radio frequency (RF) signals on a plurality of subcarriers forwireless communications. Associated with each of the plurality of basestations 110 is a coverage area 125 wherein the wireless communicationdevice 120 can receive OFDMA signals from and transmit signals to one ormore of the plurality of base stations 110. The wireless communicationdevice 120 will typically receive signaling and other messaging from abase station having the strongest signal strength, or otherwise somepreferable signal characteristics such that the particular base station110 is the “best server” to the particular wireless communication device120. The plurality of base stations 110 are coupled to a network systemcontroller 130 for centralized control of the OFDMA wirelesscommunication system.

An OFDMA wireless communication system is a multi-carrier modulationscheme which has been proposed as a next generation solution for presentwide-area code division multiple access (WCDMA) wireless communicationsystems. OFDMA is a more general case of an orthogonal frequency domainmultiplexing (OFDM) system wherein data for different users can betransmitted simultaneously on different subcarriers. OFDMA wirelesscommunication systems have a large number of subcarriers, wherein asubcarrier only occupies a small fraction of the OFDMA channel bandwidth(e.g., fifteen kilohertz (kHz) per subcarrier in a five megahertz (MHz)OFDMA channel bandwidth). Thus, for example, in a five MHz range, therecould be approximately three hundred subcarriers. OFDMA system designprovides a highly scalable, multiple system bandwidth solution because,as OFDMA systems are designed to work in different bandwidths, moresubcarriers can be added as needed. In addition, the OFDMA system designbeing contemplated for next-generation evolution of the WCDMA systemsupports both a synchronized system and an unsynchronized system andallows for a large number base station identifiers (cell index) andOFDMA symbol structures with both short and long cyclic prefix lengths.

An OFDMA system in accordance with the embodiment of the presentinvention defines a synchronization channel which significantly reducesthe time required for a wireless communication device 120 to synchronizeto the OFDMA system by acquiring the OFDMA system timing simultaneouswith identifying the strongest base station 110, or “best server” asdescribed above, for establishing communication therewith (i.e., theinitial acquisition and cell search time). The OFDMA initial acquisitionand cell search process should detect an OFDMA symbol timing, a frameboundary and a frequency error as well as detect cell specificinformation such as an identification of the base station 110 and, ifnecessary, other cell specific information such as the system bandwidth,the number of transmission antennas on the base station 110 or a cyclicprefix length. The synchronization signal in accordance with theembodiment of the present invention includes at least partial cell(i.e., base station) identification information. The cell identificationinformation of the synchronization channel could be partial cellidentification information identifying a group of individual basestations 110 (e.g., cell group identification information) or could befull cell identification information identifying a unique base station110, and may further provide sector identification information inembodiments in which base stations 110 are partitioned by antennacoverage patterns and resource allocation into multiple sectors.

Referring to FIG. 2, an exemplary OFDMA frame structure depicts a singleOFDMA frame 200 of ten milliseconds transmission time comprising onehundred and forty OFDMA symbols. The frame 200 includes twentysub-frames 210, 220, where the first sub-frame 210 is thesynchronization channel occupying a seven OFDMA symbol sub-frame 210where the seven OFDMA symbols 230 form a short cyclic prefix (CP)sub-frame. The remaining nineteen sub-frames 240 can either be a long CPsub-frame having six OFDMA symbols 240 or a short CP sub-frame havingseven OFDMA symbols 230. While the example in FIG. 2 depicts thesynchronization channel in a first sub-frame 210 having a short cyclicprefix, location of the synchronization channel and the cyclic prefixthereof can be defined in any manner or location to accommodate theOFDMA system design. By locating the synchronization channel in thefirst sub-frame 230 (as shown) or the last sub-frame in accordance withanother embodiment of the present invention, the frame boundary isdefined by the synchronization channel.

Referring to FIG. 3, an OFDMA frame structure in accordance with analternate embodiment of the present invention is depicted. In accordancewith this alternate embodiment, the synchronization channel 310 isassigned to the end of more than one of the twenty sub-frames 320 inorder to detect the synchronization channel 310 regardless of the CPlength. The synchronization channel 310 is transmitted every N sub-frame320 in order to reduce the initial acquisition and cell search time andmemory size of initial acquisition in unsynchronized OFMDA systems,where N is an aliquot of twenty. It will be recognized by those skilledin the art that the system parameters of the sub-frames, the length andnumber of symbols of the OFDMA system frame and other frame structureparameters may be modified in accordance with a plurality of systemdesigns, and the frame structure of an OFDMA system in accordance withthe present invention is not restricted to the embodiments of FIG. 2 orFIG. 3.

The synchronization channel, in accordance with an embodiment of thepresent invention, is transmitted within a localized portion of thebandwidth of the OFDMA signal, e.g., the center 1.25 MHz bandwidth ofthe OFDMA signal, regardless of the system bandwidth, thereby reducingthe initial acquisition and cell search time while preserving thescalability of the OFDMA wireless communication system. Referring toFIG. 4, predetermined resource blocks 410 are predefined frequencybands. While it is recognized that any frequency band can be defined forthe resource blocks, in accordance with one embodiment of the presentinvention, the resource block (RB) size is 0.375 MHz and thesynchronization channel 420 is generally defined to be 1.5 MHz, thusoccupying four resource blocks 410. Subcarrier symbols in systembandwidth except for the center resource blocks 410 occupied by thesynchronization channel 420 are utilized for other channels. In anotherembodiment, the bandwidth of the synchronization channel is related tothe OFDMA signal bandwidth. Some examples of this are OFDMA systembandwidths 430, 440, 450, 460, 480.

In a twenty megahertz OFDMA system 430 (having forty-eight resourceblocks 410) and a ten megahertz OFDMA system 440 (having twenty-fourresource blocks 410), the synchronization channel 420 uses the centraltwelve resource blocks 410. In a five megahertz OFDMA system 450 (havingtwelve resource blocks 410), the synchronization channel 420 uses alltwelve resource blocks 410. In a 2.5 MHz OFDMA system 460 (having sixresource blocks 410), the synchronization channel 420 uses only thecentral four resource blocks 410. Utilizing the symmetry of thesynchronization channel 420, the spectrum 470 of the synchronizationchannel 420 covers the central portion of the four resource locks 410 ofthe synchronization channel 420. Unused subcarriers on either side ofthe synchronization channel spectrum 470 can be used for guard bands ordata (e.g., low rate channels such as acknowledgements of receiveduplink traffic, or other data streams/channels).

In another embodiment where the bandwidth of the synchronization channelis related to the OFDMA signal bandwidth, the synchronization channelsignal may be repeated in the frequency dimension to further improveperformance. For example, the synchronization channel signal informationmay be contained in the central four resource blocks. Then, eachadditional set of four resource blocks that are within thesynchronization channel bandwidth may contain another transmission orrepetition of the synchronization channel signal contained in thecentral four resource blocks.

In addition to the partial or full cell identification information orthe repetition or transmission of the synchronization channel signal,for five megahertz or larger bandwidth OFDMA systems, thesynchronization channel 420 can use frequency bands other than thecenter four resource blocks to enhance cell search performance. Forexample, all or a portion of additional cell specific information suchas frequency reference information, transmission antenna information,pilot stream information or cyclic prefix (CP) length information couldbe included in the synchronization channel 420 information. In addition,the OFDMA system could be designed to redundantly transmit thesynchronization channel on two or more of a plurality of subcarriers theportion of bandwidth occupied by the synchronization channel 420.

For the case where the OFDMA system bandwidth is 1.25 MHz 480, onlythree resource blocks 410 can be accommodated and the synchronizationchannel 420 uses all three resource blocks 410. While a number ofvariations of OFDMA system bandwidth have been shown, other structuresare possible wherein the synchronization channel is transmitted in alocalized portion of the OFDMA system bandwidth.

FIG. 5 depicts a five megahertz OFDMA communication system signalbandwidth where the localized synchronization channel bandwidth 510 islocated in the center 1.25 MHz of the five megahertz bandwidth andwithin, but smaller than a bandwidth spanned by a multiple number ofresource blocks 520. In this instance, the synchronization channelbandwidth 510 does not cover a multiple of the resource block size 520.In accordance with the embodiment of the present invention, a datasignal 530 is transmitted simultaneously with the synchronizationchannel in a portion of the bandwidth spanned by an integer number ofresource blocks 520 that is not utilized by the synchronization channel510. For improved detection of the data signal 530, it may be separatedfrom the synchronization channel by bandwidths where no information istransmitted called guard bands 540.

The synchronization channel signal is a sequence divided intosynchronization channel signal sequence elements. An example of apreferred sequence type in accordance with the present invention is ageneralized chirp like (GCL) sequence. For example, a length-N_(G) GCLsequence of “index” u which is defined as

S _(k) =a _(k) b,k=0, . . . ,N _(G)−1  (1)

where b is a complex scalar of unit amplitude and

$\begin{matrix}{{a_{k} = {\exp \left( {{- {j2\pi}}\; u\frac{{{k\left( {k + 1} \right)}/2} + {qk}}{N_{G}}} \right)}},{k = 0},1,2,\ldots \mspace{14mu},{N_{G} - {1\left( {{{any}\mspace{14mu} {integer}\mspace{14mu} q\mspace{14mu} {and}\mspace{14mu} 1} \leq u \leq {N_{G} - 1}} \right)}}} & (2)\end{matrix}$

and N_(G) is a prime number (i.e., N_(G)=N_(G)×1) is particularlysuitable for a sequence divided into synchronization channel signalsequence elements in accordance with the present invention. Where N_(G)is a prime number, the cross-correlation between any two sequences ofdistinct “class” is optimal and there will N_(G)−1 unique sequences inthe set that can be used as unique group identifiers or unique cellidentification information. The GCL sequence can be represented moresimply and compactly by choosing b=1 and q=0.

Additional examples of sequence types that can be used for thesynchronization channel sequence elements in accordance with the presentinvention may include a Pseudo-random Noise (PN) sequence or a maximallength binary sequence. When a structured sequence with limited choicesof sequence length (such as GCL or maximal-length binary) is used, thenumber of elements in the original sequence may not match size of thesynchronization channel. In this case, the sequence may be modified tofit within the resources available for the synchronization channelsignal sequence (e.g., by truncation or cyclic extension thereof). Inaccordance with another aspect of the embodiment of the presentinvention, the synchronization signal includes a plurality ofsynchronization channel signal sequence elements that are distributedover the OFDMA signal subcarriers and/or the OFDMA symbol periods asdetermined by the OFDMA system design or by signal propagationconditions that the system is expected to operate in.

FIG. 6, comprising FIGS. 6A, 6B and 6C, depicts frame structures forsynchronization channel sequence element assignment in accordance withthe present invention wherein the synchronization channel sequenceelements are distributed over frequency (the subcarriers) first and thenover time. The present invention, however, is not limited to thissynchronization channel sequence element assignment scheme and mayalternatively distribute the synchronization channel sequence elementsover time first and then frequency if, for example, the system designallows changes in time faster than in frequency. Referring to FIG. 6A,the synchronization channel signal is transmitted over a sub-frame 610with a frame structure of seven OFDMA symbols, wherein thesynchronization channel sequence elements are transmitted on a pluralityof subcarriers in adjacent or proximal OFDMA symbol periods. While notshown, in some embodiment's pilot symbols or other symbols such ascontrol symbols may occupy part or all of one or more of the OFDMAsymbol periods in sub-frame 610, such that the time spacing between someof the aforementioned proximal OFDMA symbol periods may be more than oneOFDMA symbol period.

In accordance with the present invention, a first OFDMA symbol period620 includes a common GCL sequence of modulation symbols or zerosforming thirty-eight sequence elements mapped onto thirty-eightsubcarriers, the GCL sequence in the first OFDMA symbol period 620 beingcommon for all of the base stations 110 in the OFDMA wirelesscommunication system 100. By using every other sub-carrier (e.g., evennumbered subcarriers) for this common GCL sequence 620, the waveform canhave a predetermined time domain symmetry. This common GCL sequence 620may be present in all synchronization channel transmissions and may belocated in the first OFDMA symbol period of the sub-frame 610, therebyutilized as a frame boundary indicator. Referring to FIG. 7, an exampleof the sub-carrier mapping of the synchronization channel signal in thefirst OFDMA symbol period 620 is shown where modulated symbols aremapped to every other subcarrier (the thirty-eight occupied subcarriers702) with the intervening subcarriers 704 having zeros or null setsmapped thereto. The modulation symbols are mapped to even numberedsubcarriers in order to create or define the symmetry of the waveform inthe time domain (i.e., the predetermined time domain symmetry of thesynchronization channel signal waveform). This symmetry characteristiccan be utilized for coarse OFDMA symbol timing detection and frequencyerror detection.

Referring back to FIG. 6A, the subsequent six OFDMA symbol periods 630include the GCL sequence unique to a group of cells or base stations, orunique to the cell or base station 110 (depending on the embodiment) asa plurality of synchronization channel sequence elements mapped onto aplurality of subcarriers, each OFDMA symbol period having allseventy-five subcarriers used for the GCL synchronization channelsequence elements and filling the six OFDMA symbol periods 630 in a“zig-zag” fashion. For example, FIG. 6A depicts the synchronizationchannel signal GCL sequence including 449 synchronization channelsequence elements. The second OFDMA symbol period 630 is filled withsynchronization channel signal sequence elements (phases) 0 to 74ordered from top to bottom. The third OFDMA symbol period 630 is filledwith synchronization channel signal sequence elements 75 to 149 orderedfrom bottom to top, but in an alternate embodiment could also be orderedfrom top to bottom. In a like manner, the remaining OFDMA symbol periods630 are filled with the remaining synchronization channel signalsequence elements, with the sixth OFDMA symbol being filled withsynchronization channel signal sequence elements (phases) 375 to 449ordered from bottom to top. Instead of filling the OFDMA symbol periodsof the synchronization channel in a “zig-zag” fashion, the OFDMA symbolperiods 630 could all be filled from top to bottom or vice versa inaccordance with the OFDMA system design, the sequence type and/or theprocessing necessary to combine the synchronization channel sequenceelements. In addition, instead of filling the synchronization channel ina frequency-first fashion, the OFDMA symbol periods 630 could be filledin a time-first fashion (e.g., from left to right on each subcarrier,right to left on each subcarrier, or left to right on some subcarriersand right to left on other subcarriers). Or, instead of the abovedescribed filling methods, any arbitrary two-dimensional filling patterncould be used.

Referring to FIG. 6B, a synchronization channel signal unique to a cellor base station 110 or a group of cells (e.g., a GCL sequence common tomultiple cells) is also transmitted over a sub-frame 610 with a framestructure of seven OFDMA symbols, wherein the synchronization channelsequence elements are transmitted on a plurality of subcarriers inadjacent or proximal OFDMA symbol periods. In accordance with thisembodiment of the present invention, the first OFDMA symbol period 620includes zeros mapped onto 37 subcarriers and elements of acell-specific or group-specific GCL sequence forming thirty-eightsequence elements mapped onto thirty-eight subcarriers, for one or agroup of the base stations 110 in the OFDMA wireless communicationsystem 100. The subsequent six OFDMA symbol periods 630 includeadditional elements of the cell-specific GCL sequence mapped onto aplurality of subcarriers, each OFDMA symbol period having allseventy-five subcarriers (phases), filling the six OFDMA symbol periods630 in the “zig-zag” fashion. FIG. 6B depicts the synchronizationchannel signal GCL sequence including 487 synchronization channelsequence elements. The second OFDMA symbol period 630 is filled withsynchronization channel signal sequence elements (phases) 38 to 112ordered from bottom to top. The third OFDMA symbol period 630 is filledwith synchronization channel signal sequence elements 113 to 187 orderedfrom top to bottom. In a like manner, the remaining OFDMA symbol periods630 are filled with the remaining synchronization channel signalsequence elements, with the sixth OFDMA symbol being filled withsynchronization channel signal sequence elements (phases) 413 to 487ordered from top to bottom.

Referring to FIG. 6C, another alternate structure of a synchronizationchannel sequence assignment is shown. In accordance with the presentinvention, the synchronization channel sequence elements may bedistributed over the OFDMA symbol periods (as shown in FIG. 6A) or maybe distributed over more than one of the plurality of subcarriers of theOFDMA signal, or a combination of both distributions. In the alternateembodiment of FIG. 6C, there are ten synchronization channel symbolperiods in the frame structure 640. In order to accommodate a longercommon GCL sequence (e.g., longer than thirty-eight sequence elements),a first portion 650 of the synchronization channel includes two OFDMAsymbol periods 660, 670. The first OFDMA symbol period 660 may be usedas a frame boundary indicator. In accordance with the alternateembodiment of the present invention, the synchronization channelsequence elements are mapped to every second sub-frame such that thefirst synchronization channels 650, which includes seventy-fivesubcarriers, is mapped to the first OFDMA symbol period 660 and thesecond OFDMA symbol period 670. Each of the OFDMA symbol periods 660,670 with the common GCL sequence includes thirty-eight sub-carriers,where the use of even numbered sub-carriers maintains the predeterminedtime domain symmetry of the synchronization channel as shown in FIG. 7and discussed above.

Channel conditions could change during a gap between the sub-frames. Toaccommodate the differential processing of the synchronization channelsequence elements, the subsequent OFDMA symbol period 670 may repeat, asshown in FIG. 6C, the last sequence element (e.g., phase 37) of theprevious OFDMA symbol period 620. Following the first synchronizationchannels 660, the second synchronization channels 680 include eightOFDMA symbol periods having 592 synchronization channel sequenceelements mapped to seventy-five subcarriers for each OFDMA symbolperiod. The eight OFDMA symbol periods 680 for the secondsynchronization channels use every second sub-frame and are filled in a“zig-zag” fashion (as shown) or any arbitrary two-dimensional fillingpattern as discussed above, repeating the last sequence element of anOFDMA symbol period as the first sequence element of the next OFDMAsymbol period. Accordingly, the third OFDMA symbol period is filled withsynchronization channel signal sequence elements (phases) 0 to 74ordered from top to bottom. The fourth OFDMA symbol period is filledwith synchronization channel signal sequence elements 74 to 148 orderedfrom bottom to top.

Within each synchronization channel sequence element, GCL sequenceelements may preferably be employed such that differential processing ofthe GCL sequence elements will provide determination of the sequenceindex. GCL sequence elements have 0 dB peak-to-average power ratio(PAPR) and optimal cross correlation properties. If a GCL sequence isapplied in the frequency domain on all subcarriers, the properties stillhold for the corresponding time-domain waveform since the Fouriertransform of a GCL sequence is also a GCL sequence. In addition, if aGCL sequence is passed through a differential demodulator, the resultingoutput sequence is a complex exponential with a frequency thatcorresponds to the original sequence index. Thus, using GCL sequenceelements, each synchronization channel signal sequence element will havesequence index properties for inherently determining the sequence indexthereof. As mentioned earlier, other types of sequences could also beused, but it is preferred that the sequence have properties that enablesequence index detection based on the differential demodulation of thesequence. One example of a sequence other than GCL that has suchproperties is a maximal-length binary sequence, since a differentialdemodulation of a maximal-length binary sequence produces a cyclicallyshifted version of the same sequence with a predetermined shift value.Thus, with a maximal-length binary sequence, each cell ID can beassociated with a particular cyclic shift value of the sequence, and thecell ID can be recovered based on differential processing.

Referring to FIG. 8, a block diagram of the OFDMA base station 110includes a base station controller 810 coupled to the network controller130 and controlling the operation of the base station 110. Thecontroller is coupled to receiver circuitry 812 and transmittercircuitry 814, and may further include a receiver/transmitter switch 816for controlling the transmission and reception of the OFDMA signals overthe antenna 818 if communications over the antenna 818 are duplexed.OFDMA signals received by the receiver circuitry 812 are demodulatedthereby and provided to the controller 810 for decoding thereof. Inaddition, the controller 810 provides signals to the transmittercircuitry 814 for modulation thereby and transmission therefrom. While asingle antenna 818 is shown, it is to be understood that base stations110 may be, and are typically, configured into sectors and may employmultiple antennas for receive diversity, and/or transmission beamformingapplications, space time coding, multiple input multiple output (MIMO),or other system design transmission signaling schemes. Therefore, manytransmit and receive antenna configuration are possible in variousembodiments and FIG. 8 is not intended to be a complete schematicrepresentation of such antenna configurations but rather to exemplifycomponents helpful toward understanding the embodiments disclosedherein. With multiple antennas, it is useful to convey the number ofantennas to the wireless communication devices 120 to know how manypilot streams to search for during initial acquisition and cell search.Thus, in accordance with an embodiment of the present invention, theadditional cell specific information that may be transmitted as part ofthe synchronization channel signal may include the number of antennas ofthe base station 110 or pilot stream information. The controller 810 iscoupled to a storage device 820 which stores information for theoperation of the base station 110 such as cell identificationinformation and other cell specific information such as frequencyreference information, transmission antenna information (such as thenumber of antennas), pilot stream information and cyclic prefix lengthinformation.

In accordance with the present invention, the controller 810 includes asynchronization channel generator 822 for generating a synchronizationchannel signal having time domain symmetry within a portion of the OFDMAsignal bandwidth and comprising at least partial cell identificationinformation, the synchronization channel generator 822 providing thesynchronization channel signal to the transmitter circuitry 816 fortransmission therefrom. Sometimes the synchronization channel generator822 generates a synchronization channel signal including at least aportion of additional cell specific information. A data signal generator824 generates an OFDMA data signal for providing to the transmittercircuitry 816 for transmission therefrom and, in accordance with oneaspect of the present invention wherein the bandwidth is divided into aset of resource blocks, the data signal is transmitted simultaneouslywith the synchronization channel signal on a portion of a bandwidthspanned by an integer number of predetermined resource blocks when thesynchronization channel signal spans a bandwidth smaller than abandwidth spanned by the integer number of predetermined resourceblocks. Data could be voice or MBMS transmissions that are generated bya calling wireless communication device 120 or by a content provider andmay be multiplexed onto the subcarriers and interleaved at the basestation 110 or multiplexing may be performed by the network controller130. The synchronization channel generator 822 defines the time domainsymmetry of the synchronization channel signal in one embodiment bymapping modulation signals and zeros onto a plurality of subcarriersthereof.

Referring to FIG. 9, operation of the synchronization channel generator822 in accordance with the embodiment of the present invention begins byretrieving information 910 from the storage device 820. At a minimum,this information includes cell identification information uniquelyidentifying the base station 110 or at least partial cell identificationinformation, such as group cell identification information. Additionalcell specific information, as discussed above, could also be retrieved910.

Next, the synchronization channel signal is generated 912 by encodingthe cell identification information. The synchronization channel signalis parsed into a plurality of synchronization channel sequence elements914. The predetermined time domain symmetry of the synchronizationchannel signal is then defined 916. In accordance with the presentinvention, step 916 would include providing an even number ofsubcarriers in a resource block and may include mapping the generatedsynchronization channel signal as modulation symbols and zeros onto aplurality of subcarriers where the modulation symbols are mapped toevery nth subcarrier of at least a portion of the subcarriers utilizedfor the synchronization channel signal, where n is an integer greaterthan or equal to two.

After the time domain symmetry is defined 916, the synchronizationchannel signal is provided 918 to the transmitter circuitry 816 fortransmission from the base station 110. The synchronization channelsignal is periodically transmitted from the base station 110 to enableinitial acquisition and cell search. Thus, the synchronization channelsignal may, in addition to the foregoing be provided to the transmittercircuitry 816 redundantly either in time or across subcarriers forimproved initial acquisition and cell search. The redundancy and thecontent of the synchronization channel signal can be revised and/orredefined based upon the bandwidth of the OFDMA signal (i.e., inresponse to the scaling of the OFDMA signal bandwidth).

Referring to FIG. 10, a wireless communication device 120 in accordancewith the embodiment of the present invention is shown. The wirelesscommunication device 120 includes an antenna 1002 for receiving andtransmitting radio frequency (RF) signals. A receive/transmit switch1004 selectively couples the antenna 1002 to receiver circuitry 1006 andtransmitter circuitry 1008 in a manner familiar to those skilled in theart. The receiver circuitry 1006 demodulates and decodes the RF signalsto derive information therefrom and is coupled to a controller 1010 forproviding the decoded information thereto for utilization thereby inaccordance with the function(s) of the wireless communication device120. The controller 1010 also provides information to the transmittercircuitry 1008 for encoding and modulating information into RF signalsfor transmission from the antenna 1002. While a single antenna 1002 isdepicted, those skilled in the art will recognize that diversityantennas could be used with diversity receivers for improved signalreception.

The controller 1010 is coupled to user interface circuitry 1012including, for example, a display for presenting video output to a user,a speaker for providing audio output to the user, a microphone forreceiving voice input, and user controls, such as a keypad, forreceiving user input thereby. The controller 1010 is further coupled toa nonvolatile memory device 1014 for storing information therein and forretrieving and utilizing information therefrom.

In accordance with the embodiment of the present invention, the receivercircuitry 1006 includes a synchronization channel signal filter device1016 for isolating a portion of the OFDMA signal bandwidth whichincludes the synchronization channel signal. The synchronization channelsignal filter device 1016 could be a bandpass filter or any other deviceor process for filtering the OFDMA signal to isolate a localized portionof the OFDMA signal bandwidth. For example, a fast Fourier transform(FFT) could be utilized to isolate the localized portion of the OFDMAsignal bandwidth during processing instead of a hardware filter. Onceisolated, the signal is provided to the controller for initialacquisition and cell search processing.

Referring to FIG. 11, the initial signal acquisition and cell searchprocess begins by examining the signal filtered by the filter 1016 todetermine if there is any signal 1110. When a signal is detected 1110,the initial acquisition and cell search method is performed inaccordance with the present invention. First, the predetermined timedomain symmetry of the synchronization channel signal is utilized toperform coarse OFDMA symbol timing detection and fractional frequencyoffset detection 1112. This step 1112 could be performed by differentialcorrelation of the received synchronization channel signal beingcalculated in the time domain or by correlation calculation with knownsynchronization channel signal sequence elements in the time domain.

Generalized chirp like (GCL) sequences are preferably suited todifferential processing in accordance with the embodiment of the presentinvention. However, as mentioned previously, the present invention canuse other sequence types. The time domain waveforms of the GCL-modulatedOFDM signals have low PAPR. In addition, because of the use of differentindices of the GCL sequences, any pair of the sequence elements willhave low cross correlation at all time lags, which improves the codedetection and CIR estimation. Also, GCL sequences have constantamplitude, and the N_(G)-point DFT of GCL sequences also have constantamplitude. GCL sequences of any length additionally have an “ideal”cyclic autocorrelation (i.e., the correlation with the circularlyshifted version of itself is a delta function). And, the absolute valueof the cyclic cross-correlation function between any two GCL sequencesis constant and equal to 1/√{square root over (N_(G))}, when |u₁−u₂|,u₁, and u₂ are all relatively prime to N_(G) (a condition that can beeasily guaranteed if N_(G) is a prime number).

The cross-correlation 1√{square root over (N_(G))} at all lags actuallyachieves the minimum cross-correlation value for any two sequenceelements that have the ideal autocorrelation property (meaning that thetheoretical minimum of the maximum value of the cross-correlation overall lags is achieved). The minimum is achieved when the crosscorrelations at all lags is equal to 1/√{square root over (N_(G))}. Thecross correlation property allows the impact of an interfering signal beevenly spread in the time domain after correlating the received signalwith the desired sequence in the time domain. Hence, the cell-searchsymbol can also be used to perform or assist coherent channel estimationat the wireless device even before the broadcast pilot symbols areprocessed. Compared with BPSK or even QPSK preambles, the complex-valuedGCL sequences can be systematically constructed with guaranteed goodPAPR and good correlation.

Differential processing of the GCL sequence elements enables the onestep fast cell search for GCL sequence elements, step 1112. Tofacilitate differential processing in accordance with the embodiment ofthe present invention, the sequence elements have preferably beengenerated in accordance with a sequence design methodology for asequence length N_(p) where a prime number N_(G) is the smallest primenumber larger than N_(p). The integer “u” is the sequence index. Thesequence elements were generated according to

$\begin{matrix}{{{{s_{u}(k)} = {\exp \left\{ {{- {j2\pi}}\; u\frac{k\left( {k + 1} \right)}{N_{G}}} \right\}}},{k = {{0\mspace{14mu} \ldots \mspace{14mu} N_{G}} - 1}},{and}}{u = {{1\mspace{14mu} \ldots \mspace{14mu} N_{G}} - 1}}} & (1)\end{matrix}$

N_(G)−1 sequence elements are generated having an optimal cyclic crosscorrelation between any pair of them. The sequence elements have beentruncated to N_(p) and distributed over N_(p) subcarriers. Due to theoversampling introduced in OFDMA signaling with null subcarriers, andalso the use of localized bandwidth for the synchronization signal, thePAPR will be degraded to different degrees for different “u” from thetheoretical 0 dB value (at Nyquist sampling rate). If desired, indicesthat have the best PAPR among N_(G)−1 candidates can be chosen. The cellsearch sequences used by different cells are obtained from differentindices “u” of these GCL sequence elements. The index “u” will also actas a cell ID.

The cell search 1112 determines directly the sequence indices “u” (andhence the strongest or candidate cell ID's or group ID's) from thereceived signal. First, the coarse OFDMA cell-search symbol timing isdetermined (e.g., using the time domain symmetry of the cell-searchsymbol). Then, the fractional part of the frequency offset is estimatedand removed (e.g., based on the phase of the half-symbol differentialcorrelation peak). After these steps, a block of N received time-domainsamples representing the received cell-search symbol is transformed tothe frequency domain using the usual FFT process.

Assuming that an integer frequency offset may still be present, theoccupied subcarriers (even vs. odd) can be determined next by varioustechniques such as a maximum energy detector (e.g., total energy in theeven subcarriers of the cell-search symbol vs. energy in the oddsubcarriers). The frequency domain data on the occupied subcarriers asY(m) for m=1 to N_(p) (i.e., ignoring the unused subcarriers) is denotedwhere S_(u)(m) is the GCL sequence mapped onto those subcarriers.

Next, a vector of “differential-based” values is computed based on thepairs of occupied subcarriers. These values, which are obtained bydifferentially demodulating the occupied subcarriers of the receivedsymbol, are conveniently collected into vector format (e.g., adifferential-based vector) for efficient FFT-based processing. Thedifferential-based vector is computed as

Z(m)=Y(m)*Y*(m+1),m=1, . . . ,N _(p)−1.  (2)

where “( )*” denotes conjugation. Other ways to obtain the“differential-based” vector may include, but are not limited to:

Z(m)=Y(m)/Y(m+1),m=1, . . . ,N _(p)−1.  (3)

or

Z(m)=Y(m)/Y(m+1)/abs(Y(m)/Y(m+1)),m=1, . . . N _(p)−1.  (4)

where “abs( )” denotes the absolute value.

Assuming that there is only one base station, and that it istransmitting a cell-search symbol with a GCL sequence index of u, andthat the channel does not change significantly between two adjacentoccupied subcarriers, which is approximately satisfied as long as thespacing of occupied subcarriers is not too large, ignoring the channelamplitude and frequency offset, Y(m)*Y*(m+1) is approximately equal to

$\begin{matrix}{{\left. {{Z(m)} = {{{Y(m)}*{Y^{*}\left( {m + 1} \right)}} \approx {{{H(m)}}^{2}{S_{u}(m)}{S_{u}^{*}\left( {m + 1} \right)}}}} \right) = {{{H(m)}}^{2}\exp \left\{ {{j2\pi}\; u\; \frac{m + 1}{N_{G}}} \right\}}},\mspace{79mu} {m = 1},\ldots \mspace{14mu},{N_{p} - 1.}} & (5)\end{matrix}$

Thus, the sequence index information u is carried in thedifferential-based vector. In the multi-cell case, by processing thedifferential-based vector and identifying a set of prominent frequencycomponents of the vector, we can identify the strongest cell index andone or more indices of potential handoff candidates as well. To obtainthe frequency domain components, a commonly used tool is to take an FFTor IFFT (say T-point, T>=N_(p)−1) on {Z(m)} (step 1114) to get

{z(n)}=IFFT _(T)({Z(m)}),m=1, . . . ,N _(p)−1,n=1, . . . ,T.  (6)

The peak position (say n_(nax)) of {z(n)} gives information about thestrongest cell's index u, i.e., the mapping between the identifiedprominent frequency component at n_(max) to a corresponding transmittedsequence index is determined as

$\begin{matrix}{\frac{u}{N_{G\;}} = {\frac{n_{\max}}{T}.}} & (7)\end{matrix}$

The peak values are also rough estimates of the channel power at theoccupied subcarriers. Thus, IFFT of the synchronization channel signalin the frequency domain is used to detect the frame boundary and decodethe cell identification information 1114. Thus, utilizing the sequenceindex properties of the synchronization channel sequence elements,multiplying one sequence element by the complex conjugate of a nextsequence element will derive the sequence index u 1114. Accordingly, ina single step, the controller 1010 can perform GCL sequence index detectto extract the cell specific information (e.g., u) from thesynchronization channel signal. When the synchronization channel signalis determined to be, in some embodiments, the strongest synchronizationchannel signal 1116, wireless OFDMA communication is established withthe base station 1118.

Note that for the purpose of explanation, the above equations weredescribed for the case of the GCL sequence elements being mapped todifferent subcarriers of one OFDMA symbol period. However, the proposeddetection method can also be applied when the sequence is mapped inother ways, such as “zig-zag”. In general, the differential demodulationstep can be performed over adjacent sequence elements even if theadjacent sequence elements are mapped to different OFMDA symbol periodsand/or different subcarriers. In addition, the differential processingfrom multiple received instances of the synchronization channel can becombined to further improve the detection robustness. Multiple receivedinstances of the synchronization channel may be available due to eitherreceive diversity with multiple antennas, or from subsequently receivedsynchronization signals that are transmitted periodically by the basestation, for example.

As described for some embodiments of the invention, the time domainsymmetry of the synchronization signal can be provided by mappingmodulation symbols or sequence elements to even-numbered subcarriers inthe localized synchronization channel bandwidth and zeros to othersubcarriers in the localized synchronization channel bandwidth. Otherembodiments of the invention may utilize other methods for providingtime domain symmetry. One example includes mapping modulation symbols orsequence elements to every Nth subcarrier in the localizedsynchronization channel bandwidth and zeros to the other subcarriers inthe localized synchronization channel bandwidth, where N is a positiveinteger, and where the subcarrier in the localized synchronizationchannel bandwidth containing the first of the every Nth subcarrier canbe arbitrarily chosen. An additional example is to use modulationsymbols or sequence elements that are purely real (i.e., their imaginarypart is zero) in the localized synchronization channel bandwidth, sincethe Fourier transform of a real signal is symmetric in magnitude aroundits central portion. Methods of sequence design and/or mapping and/orsignal repetition other than the provided examples can also be used toprovide predetermined time domain symmetry.

Thus, it can be seen that the present invention provides an initialacquisition and cell search method utilizing interleaved synchronizationchannel signal sequence elements with low computational load and a smallnumber of receiver processing steps which nevertheless provides the fourmain functions of initial acquisition and cell search (i.e., OFDMAsymbol timing detection, frequency error detection, frame boundarydetection and cell specific information detection) in an OFDMA systemsupporting multiple system bandwidths, both synchronized andun-synchronized systems, a large cell index and an OFDMA symbolstructure with both short and long cyclic prefix length. While at leastone exemplary embodiment has been presented in the foregoing detaileddescription of the invention, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or exemplary embodiments are only examples, and arenot intended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the foregoing detailed description willprovide those skilled in the art with a convenient road map forimplementing an exemplary embodiment of the invention, it beingunderstood that various changes may be made in the function andarrangement of elements described in an exemplary embodiment withoutdeparting from the scope of the invention as set forth in the appendedclaims and their equivalents.

What is claimed is:
 1. A method for use in an orthogonal frequencydomain multiple access (OFDMA) base station in a wireless communicationsystem including a synchronization channel generator and transmittercircuitry, the method comprising the steps of: generating asynchronization channel signal by the synchronization channel generator,the synchronization channel signal comprising a plurality ofsynchronization channel signal sequence elements including a first setof synchronization channel signal sequence elements, wherein the firstset of synchronization channel signal sequence elements is based on acyclically shifted maximal length binary sequence with a cyclic shift ofthe maximal length binary sequence defined in response to cellidentification information associated with the base station; andtransmitting an OFDMA signal by the transmitter circuitry, the OFDMAsignal including the synchronization channel signal.
 2. The method ofclaim 1, wherein transmitting the OFDMA signal including thesynchronization channel signal comprises transmitting thesynchronization channel signal has time domain symmetry within alocalized portion of bandwidth within which the synchronization channelis transmitted.
 3. The method of claim 1, wherein the cellidentification information includes a cell identifier.
 4. The method ofclaim 1, wherein the cell identification information includesinformation identifying the base station.
 5. An orthogonal frequencydomain multiple access (OFDMA) base station comprising: asynchronization channel generator configured to generate asynchronization channel signal by the synchronization channel generator,the synchronization channel signal comprising a plurality ofsynchronization channel signal sequence elements including a first setof synchronization channel signal sequence elements, wherein the firstset of synchronization channel signal sequence elements is based on acyclically shifted maximal length binary sequence with a cyclic shift ofthe maximal length binary sequence defined in response to cellidentification information associated with the base station; andtransmitter circuitry transmitting an OFDMA signal, the OFDMA signalcomprising the synchronization channel signal.
 6. The base station ofclaim 5, wherein transmitting the OFDMA signal including thesynchronization channel signal comprises transmitting thesynchronization channel signal has time domain symmetry within alocalized portion of bandwidth within which the synchronization channelis transmitted.
 7. The base station of claim 5, wherein the cellidentification information includes a cell identifier.
 8. The basestation of claim 5, wherein the cell identification information includesinformation identifying the base station.